Isolated ceremonial structures from the back of beyond at Nashoba Brook

Went for an extended stroll in the large undifferentiated flat area between Spring Hill and Nashoba Brook in Acton.

Here is a pair of rocks on a rock, perhaps forming a gap with the adjacent boulder; from next to the pipeline:One split-wedged rock: Here is a little prayer seat. Someone has been here recently, not clear what for:
Some views of a table rock which, I think, is another form of split-wedged rock: Another: